152 
NATURE 
[JUNE 15, 1899 
seasons generally exhausts the beds. This state of affairs | 
is one that loudly calls for reform. The wealth of unios 
that fills our rivers and streams is rapidly being destroyed | 
by ignorant and wasteful methods of pearl-hunting ; and | 
either some form of protection is important, or, if that be 
not possible, a wide diffusion of information as to better 
methods, and particularly the introduction of the tools 
used in Germany for opening unios far enough to see if 
there are pearls contained, without destroying the 
animal, which may then be returned to the water.” 
In the clearer streams of the country, the best method 
of collecting the mussels is by wading into the water 
armed with a water-telescope and a pair of spring nippers 
affixed to the end of a stick. The water-telescope | 
consists of a long, light, quadrangular tube open above, 
and shaped to fit the face (to which it is strapped), and 
closed below with a glass plate. Dressed in waterproof | 
clothing, the pearl-hunter wades along the bed of the | 
stream in a stooping posture, with the lower end of the 
tube immersed in the water, by which he is enabled to see 
the mussels on the bottom, and so to pick them out one 
by one with his nippers. Fresh-water pearls in general | 
are remarkable for their variety of tints, and nowhere is | 
| southern by that of Courmayeur. 
THE GEOLOGY OF MONT BLANC. 
M ONT BLANC and its aiguilles present some difficult 
problems to both petrologists and physical geo- 
logists ; problems, which, though they have something in 
common, are to a great extent distinct. The authors, 
however, have grappled with both. Their monograph, as 
a study of the petrography of the region, is full of 
valuable information ; but we think they have not been 
quite so successful in dealing with what it is now the 
fashion to call the tectonics. This portion no doubt 
contains much that is valuable, but the physical structure 
of the asszf of Mont Blanc has been treated too much 
as if the latter were isolated instead of being, as is 
| really the case, inseparable from the western and central 
part, perhaps even from the whole, of the Alpine chain. 
As most people are aware, the crystalline massz/ of 
Mont Blanc is defined by two well-marked troughs, 
occupied by rocks of secondary age, the more northern 
being furrowed by the valley of Chamonix, the more 
Each is bounded on 
the further side from Mont Blanc by crystalline rock, 
the former by the well-defined range of the Brevent and 
Fic. 
1.—Contact of protogine with crystalline schist 
below the Aiguille du Midi. 
P, protogine; s, crystalline 
schists 3 Cc, contact. 
the variation more marked than in those from Wisconsin. 
Although white is the most common, almost any colour, 
from pink, purple, or red, to gold, bronze, and black, may 
be met with; while even a peacock-blue pearl is on 
record. The golden and wine-coloured specimens are 
presumably from the beautiful Uzo dromas, the only 
common species with a golden or yellow interior to the 
shell. Pink appears to be the colour most highly esteemed 
in America, next to which comes red, and then black ; but 
exceptional colours, like sky-blue, command exceptional 
prices. So far as shape is concerned, the first place is 
taken by spherical pearls, after which come hemi- 
spherical, or bullet-shaped examples, while oval cr pear- 
shaped specimens follow. As regards the maximum 
prices obtained for American pearls, the statements are 
somewhat conflicting and indefinite. It seems, however, | 
to be certain that a spherical pink pearl from Tennessee 
realised 130/., while a sky-blue pearl from Caney Fork, in 
the same State, was sold in America for 190/., and 
subsequently in London for 6607. With good luck, there | 
is therefore evidently money to be made by pearl-hunting 
in the American rivers. R. L. 
NO. 1546, VOL. 60] 
i} 
the Aiguilles Rouges, the latter by one or more varied 
character, and, generally speaking, of more bedded 
aspect. 
Of these two marginal crystalline zones, the northern 
is prolonged to the valley of the Rhone, where it crosses 
just below Martigny, after which it disappears beneath 
the sedimentaries of the Western Oberland. The 
southern passes on to join the Pennine chain to the east 
of Mont Blanc. The crystalline rock, however, which 
forms this and the rest of the central massif, is more or 
less fusiform in outline. (The term “amygdaloidal” 
applied by the authors seems misleading, as its connection 
with this structure is about equal to that of Monmouth 
and Macedon.) The central part of the szass//—though 
according to them not the very highest rocks of Mont 
Blanc—consists of a granitoid rock called protogine, 
formerly said to be composed of quartz, felspar, and talc, 
and to be the most ancient in the region. The talc is 
only biotite, more or less hydrous, and the rock intrusive 
1 Recherches Géologiques et Pétrographiques sur le Massif du Mont- 
Blanc. Par Louis Duparc et Ludovic Mrazec. (Mem. de la Société de 
Physique et d'Histoire naturelle de Geneve.) Tome xxxiii. Ptie rre, 
